The present invention relates to a mass flow controller for controlling a mass flow of a fluid, particularly a mass flow controller whose valve element is operated by an electromagnetic actuator, and an electromagnetic valve suitable for such a mass flow controller.
A mass flow controller functions to measure a mass flow of a fluid flowing through a sensor pipe branched from a flow path of a valve casing with sensor coils wound around the sensor pipe. An operating voltage is applied to an electromagnetic actuator for moving a valve element in accordance with the difference between a predetermined value and a measured value, and the degree of opening a valve element is adjusted to control the mass flow of a fluid. The actuator is usually constituted by a stack of piazo elements to obtain a large valve element-driving force. Since the piazo elements can be operated simply by applying voltage without flowing electric current, heat generation can be avoided. However, it is difficult to obtain a large stroke of movement. For instance, in the case of a piazo element stack of 40 mm in thickness constituted by 400 piazo elements of 100 .mu.m in thickness, a stroke of movement is only 40 .mu.m.
A large stroke of movement can be obtained by an electromagnetic actuator usually used in an electromagnetic valve. In the electromagnetic actuator, electric current is caused to flow through a coil to generate a magnetic field, whereby a magnetic attraction force is generated in a soft magnetic body. In order to produce a large force by an electromagnetic actuator, it is necessary to generate a large magnetic field, namely it is necessary to increase the number of winding of a coil or electric current flowing through the coil. When a large magnetic attraction force is generated by flowing electric current through the coil, heat is inevitably generated from the coil, leading to temperature elevation in the coil and its yokes, and thus to temperature elevation in the valve casing. This temperature elevation in turn results in the change of temperature in the sensor coils, thereby generating errors in the measurement of the mass flow of a fluid with the sensor coils.
Sensor coils each consisting of a heat-generating resistance wire are wound around the sensor pipe on both upstream and downstream sides. The sensor coils are kept at a temperature higher than an ambient temperature (usually room temperature) by 30.degree.-60.degree. C., and the heat of the sensor coil on the upstream side is conveyed to the downstream side by a gas flowing through the sensor pipe. As shown in FIG. 6 of J. Phys. E. Sci. Instrum., Vol. 15 (1982), 215, a temperature difference between the coils on the upstream and downstream sides is measured as unbalanced bridge voltage. Alternatively, as described in Japanese Patent Laid-Open No. 61-128123, voltage applied to the sensor coils is changed such that electric current for compensating the temperature change flows through the sensor coils, thereby measuring the change of voltage in the sensor coils to determine the mass flow of a fluid flowing through the sensor pipe.
With respect to the change of ambient temperature, as described in Japanese Patent Laid-Open No. 1-150817, a bridge circuit is provided with a resistance for measuring an ambient temperature, keeping constant a difference between the temperature of the sensor coils and the ambient temperature, so that the change of the ambient temperature does not lead to measurement errors of the mass flow.
However, it has been found that complete compensation of the influence of the temperature change of the valve casing on the gas temperature and the measurement of the mass flow would be difficult.